This invention relates to cold traps that are employed within the flow of a liquid metal for the precipitation and removal of impurities. It is particularly applicable in sodium or sodium potassium systems used for the transfer of heat in nuclear or other power production facilities.
Oxide and hydride impurities can accumulate within a sodium-liquid-metal system from a number of sources. Small leaks of air or steam may occasionally occur and impurities may be present within the system construction materials. One major source of hydride impurities occurs from the diffusion of hydrogen through heat exchanger walls in liquid metal/steam or water heat exchangers. Heat exchangers of this type are ordinarily used as steam generators in the liquid-metal-cooled nuclear reactors. The oxidation of iron or other metals within the heat exchanger walls by H.sub.2 O releases hydrogen for diffusion through the barrier into the molten sodium.
Cold traps are most often included within a bypass or shunt loop of a liquid metal circulation system. The loop will normally include a heat exchanger called an economizer through which the diverted liquid metal flows on leaving and on reentering the main liquid-metal flow. The diverted liquid metal is cooled to some lower temperature at which the dissolved hydrides and oxides become supersaturated and precipitate as solid deposits. The deposition has normally occurred within a large crystallizer tank containing baffles and packing to facilitate the precipitation process. The returning liquid metal is reheated as it passes through the economizer in heat exchange relationship with the diverted liquid-metal flow.
The buildup of solid impurities within the cold trap crystallizer tank will eventually slow the flow of diverted liquid metal until the cold trap system is no longer effective. Heretofore, this problem was overcome by removing and replacing the crystallizer tank. A number of these tanks have been removed from various experimental liquid-metal cooled systems. Examination of a selected number of these crystallizer tanks has revealed that only approximately 10 percent of each tank's volume was filled with solid impurities. The remaining volume not occupied by baffles or packing was filled with sodium or liquid metal. The disposal of these tanks therefore results in considerable waste of materials and considerable maintenance expense in their removal and replacement.
Crystallizer tanks are presently sized to have at least a five-year life prior to their replacement. However, unforeseen air or steam leaks into the sodium flow could require earlier replacement. In primary coolant circuits that are in heat exchange relationship with a nuclear reactor core, such replacement may also become desirable to reduce the inventory of tritium and other radioisotopes deposited within the crystallizer tank.